Accuracy of Apple Watch Measurements for Heart Rate and Energy Expenditure in Patients With Cardiovascular Disease: Cross-Sectional Study

Figure 1. Heart rate (HR) measurements (bpm) by the Apple Watch are compared with gold standard electrocardiogram measurements for HRs at start (seated rest, HR1), middle (moderate intensity, HR2), and maximal performance level (HR3) of the cardiopulmonary exercise test.

Figure 2. Energy expenditure (EE) measurement (kcal) by the Apple Watch is compared with gold standard indirect calorimetry. The Bland-Altman plot compares mean values on the x-axis ((Apple Watch + gold standard)/2) with the difference of values on the y-axis (Apple Watch – gold standard) (A). Bias and limits of agreement are depicted as horizontal lines. The plot depicted in part B directly compares values measured by the Apple Watch (x-axis) versus indirect calorimetry measurements (y-axis). A systematic error is seen with an overestimation of EE by the Apple Watch.